An alkaline storage battery using a hydrogen storage alloy as the negative electrode has excellent safety and is therefore used for high power applications such as HEVs and PEVs. The hydrogen storage alloy used for such application is commonly composed of a single-phase AB2 type structure or AB5 type structure. However, recently, the hydrogen storage alloy has been required to have much higher power or much higher capacity performance than the conventional range. Accordingly, a hydrogen storage alloy including as the main phase an A2B7 type structure or A5B19 type structure in which an AB2 type structure and AB5 type structure are combined, such as a rare earth-Mg—Ni-based alloy has been proposed (see International Publication WO 2007/018292).
The crystal structure of the rare earth-Mg—Ni-based hydrogen storage alloy is transformed based on its stoichiometric ratio. That is, when the stoichiometric ratio is increased, the A5B19 type structure becomes dominant from the A2B7 type structure.
Because the A5B19 type structure has a periodically stacked structure including one layer of the AB2 type structure and three layers of the AB5 type structure, the nickel ratio per unit crystal lattice can be improved. Therefore, an alkaline storage battery using the rare earth-Mg—Ni-based hydrogen storage alloy that contains (a relatively large amount of) the A5B19 type structure as the main phase shows particularly excellent high power characteristics.
On the other hand, the high power application for HEVs, for example, commonly employs a partial charge-discharge control system in which pulse charge and discharge are repeated, for example, in the range of a state of charge (SOC) from 20 to 80%. Accordingly, in the high power application for HEVs, for example, the alkaline storage battery to be used is required to have excellent output characteristics as well as output characteristics with small variation associated with SOC variation (excellent output power stability).
Generally, the output characteristics of the alkaline storage battery containing the hydrogen storage alloy in its negative electrode closely relates to the equilibrium pressure of the hydrogen storage alloy. Specifically, the output characteristics tends to be high when the hydrogen storage alloy has a high equilibrium pressure, and the output characteristics tends to be low when the hydrogen storage alloy has a low equilibrium pressure. Consequently, when the equilibrium pressure of the hydrogen storage alloy varies associated with the SOC variation, the output characteristics vary. When the output characteristics vary associated with the SOC variation, a predetermined output power cannot be obtained in a certain SOC range. Thus, the variation of the output characteristics associated with the SOC variation is not preferable for the high power application for HEVs, for example, that requires a constant output power over from low SOC to high SOC.
Therefore, in order to reduce the variation of the output characteristics associated with the SOC variation, it is necessary to control the hydrogen storage alloy so that the equilibrium pressure varies in a small range associated with the SOC variation. That is, it is necessary to control the hydrogen storage alloy so that the variation of the equilibrium pressure is reduced in a plateau region of a PCT curve of the hydrogen storage alloy (a region typically observed in the range of an SOC of 20 to 80%, where the equilibrium pressure of the hydrogen storage alloy does not largely vary associated with the SOC variation) corresponding to a practical region.
In particular, when a rare earth-Mg—Ni-based hydrogen storage alloy having the A5B19 type structure as the main phase is used in order to obtain high output characteristics, because the crystal structure of the hydrogen storage alloy has poor stability, subphases such as an A2B7 type structure, AB5 type structure, and AB3 type structure are readily generated. Thus, the alloy has the problem that such subphases reduce the flatness in the plateau region of the PCT curve of the hydrogen storage alloy to reduce the output power stability. Therefore, when the hydrogen storage alloy is used, it should be noted that the alloy is controlled so that the variation of the equilibrium pressure in the plateau region of the PCT curve would be reduced.
Meanwhile, the reason why the subphases reduce the flatness in the plateau region of the PCT curve of the used hydrogen storage alloy as discussed above is considered as follows. Generally, when the hydrogen storage alloy is composed of a plurality of crystal structures, the PCT curve of the hydrogen storage alloy is a mixture (see FIG. 3B) of the PCT curve of each crystal structure (see FIG. 3A). However, the PCT curves are not equally mixed in all SOC regions, and mixed differently between in a low SOC region and middle to high SOC regions, and thus the finally obtained PCT curve has a tilted plateau region (see FIG. 3B).
This means that a crystal structure having a low equilibrium pressure dominantly relates to hydrogen absorption and desorption in a low SOC region and, on the other hand, a crystal structure having a high equilibrium pressure dominantly relates to the hydrogen absorption and desorption in middle to high SOC regions. Thus, it is considered that the PCT curves of the hydrogen storage alloy are mixed in the low SOC region so as to shift to the PCT curve of the crystal structure having a low equilibrium pressure and, on the other hand, the PCT curves of the hydrogen storage alloy are mixed in the high SOC region so as to shift to the PCT curve of the crystal structure having a high equilibrium pressure.
The PCT curve of each crystal structure is mixed as described above and, as a result, the plateau region of the PCT curve of the hydrogen storage alloy is tilted to have poor flatness. Therefore, it is considered that an alkaline storage battery using such a hydrogen storage alloy has a large variation in the output characteristics associated with the SOC variation to reduce the stability of the output characteristics.
Recent studies have revealed that the problem that flatness is reduced in the plateau region of the PCT curve of a rare earth-Mg—Ni-based hydrogen storage alloy to reduce the output power stability is substantially caused when the rare earth-Mg—Ni-based hydrogen storage alloy contains a large amount of La in the rare earth portion and a small amount of Al in the Ni portion. It has been also revealed that such problem can be solved by increasing the amounts of Al and the like contained in the Ni portion in the rare earth-Mg—Ni-based hydrogen storage alloy to improve the stability of the output characteristics.
In other words, the structure ratio of the AB3 type structure, AB5 type structure, and A2B7 type structure as the subphases that cause poor flatness in the plateau region of the PCT curve is controlled to be in a predetermined range. Therefore, as shown in FIG. 3C, the plateau region of the PCT curve of the hydrogen storage alloy has a small slope and high flatness, and thus it has become clear that the hydrogen storage alloy has enhanced stability of the output characteristics.
However, the Al contained in a rare earth-Mg—Ni-based hydrogen storage alloy has a standard electrode potential lower than that of Ni, and thus it suffers from the problem of easy elution in an aqueous alkali solution. Hence, a new problem has occurred when a rare earth-Mg—Ni-based hydrogen storage alloy having an increased amount of Al is used to compose an alkaline storage battery. That is, Al is eluted from the rare earth-Mg—Ni-based hydrogen storage alloy into an alkaline electrolyte during charging and discharging and moves to the positive electrode to break into positive electrode active material, and as a result, the alkaline storage battery has a reduced durability (output power durability).
Therefore, the inventors of the present invention have developed a rare earth-Mg—Ni-based hydrogen storage alloy suitable for producing an alkaline storage battery having excellent stability of the output characteristics and excellent durability and a hydrogen storage alloy electrode using the alloy for an alkaline storage battery in Japanese Patent Application 2009-210403.
It is considered that an alkaline storage battery including the hydrogen storage alloy developed in Japanese Patent Application 2009-210403 has excellent discharge performance and corrosion resistance resulting from pulverization of the hydrogen storage alloy in the initial stage of partial charge and discharge cycles. In a partial charge and discharge cycle test, a partial charge and discharge cycle is repeated at 45° C. for 6000 hours corresponding to a travel distance of 150,000 km or more or a travel time of 10 years or more of a vehicle (a partial charge and discharge cycle test for a discharge time of 3000 hours).
However, the partial charge and discharge cycle test simulating HEV control (the test repeating the cycle that discharging is stopped and charging is started when the voltage reaches an SOC of 20%, and the charging is stopped and the discharging is started when the voltage reaches an SOC of 80%) is performed to study battery durability, and as a result, it has been revealed that the battery has a problem of reduced total discharge power time (Wh: hereinafter referred to as amount of lifetime work (Wh)) that is the sum (integrated value) of the products of discharge power (W) and discharge time (h). This is thought to be because the surface of the hydrogen storage alloy is activated to improve the output power with the progress of the pulverization of the hydrogen storage alloy in the initial stage of the partial charge and discharge cycles, but associated with the progress of the partial charge and discharge cycles, the oxidation of the surface of the hydrogen storage alloy proceeds to reduce the active surface, and consequently the amount of lifetime work is reduced.